Calcium is a major nutritional requirement of vertebrate embryonic development needed particularly in the mineralization of the embryonic skeleton. In the chick embryo, the eggshell represents the extraembryonic calcium source and the placental structure, the chorioallantoic membrane (CAM), is the organ that translocates eggshell calcuim into the embryonic circulation. Previous studies by this investigator have shown the CAM calcium transport activity is a function of embryonic development and that a specific calcuim-binding protein (CaBP) is expressed in the CAM concomitantly with the onset of the transport function. The properties, the cellular location, and the mode of expression of the CaBP strongly suggest that it is functionally involved in the CAM calcium transport process. In this application, I propose to: (1) characterize the properties of the CaBP which are related to its calcium-binding activity; (2) localize the utlrastructural and subcellular site(s) of the CaBP in order to assess its functional role in calcium transport; (3) study the interaction between CaBP and other plasma membrane moieties which are potential components of the calcium transport pathway; and (4) analyze the molecular mechanisms which regulate the development-dependent expression of CaBP in the CAM. The proposal therefore represents an integrated approach to understand the cellular, biochemical, and molecular mechanisms of placental calcium transport and regulation of embryonic calcium metabolism. The information gathered here should also contribute to the understanding of prenatal nutrition in general and the underlying causes of birth defects, specifically those related to imbalance in calcium metabolism.